For the past two decades hospital outcome studies have demonstrated that hospitals with higher patient volume for particular conditions have better outcomes. Found first in studies of surgical outcome, the relationship between volume and outcome has now been demonstrated in the treatment of medical conditions. However, the explanation for the observed association between volume and outcome remains poorly understood. Many studies conclude that there is a significant unexplained "hospital effect" net of patient characteristics, physician factors, and technology use. Little attention has been given to exploring the extent to which hospital organization and staffing might account for the unexplained hospital effect despite a growing research literature showing that nurse staffing is inversely related to case-mix adjusted hospital mortality rates. This is a competing continuation of NR04513, employing data assembled as part of a cross-national study of variation in hospital outcomes. We propose to empirically test whether the volume-outcomes relationship for selected conditions, after controlling for important patient and hospital characteristics, is attenuated with the introduction of nursing variables. All hospitals in Pennsylvania (n=210) and Ontario (n=191) will be studied. Patient outcomes are derived from administrative data (30-day mortality); organizational attributes affecting clinical practice at the bedside are derived from nurse surveys; detailed hospital staffing information is derived from secondary sources and nurse surveys. This database positions us to unpack the volume-outcomes association by introducing a wide range of variables heretofore untested that characterize nursing practice, and additionally to take into consideration different degrees of regionalization of services and technology availability afforded by the Pennsylvania - Ontario comparisons. Our findings will inform policies directed toward improving hospital outcomes, and should contribute to a better understanding of the relative merits of regionalization, volume thresholds, and referral based on volume as compared to investments in clinical organization of care including the adequacy of nurse staffing.